Letters to boyfriend in osut basic training?

My boyfriend just got into army osut on Friday. I got his address and I was wondering if he will get in trouble if I write him letters in different colored ink like pink and purple. Also will he get in trouble if there is a design on the paper as long as a send it in a plain white envelope. Also can the drill sergeants open their mail? He's at fort benning, georgia I don't know if that makes a difference.